In motor vehicles, it is expected that the valves can be controlled by an electromagnetic or electromechanical type actuator.
An electromagnetic or electromechanical valve actuator usually comprises a mobile armature, called a plate or blade, and two fixed solenoids.
Such a structure is schematically illustrated by FIG. 1.
The mobile armature P moves between the two solenoids, which shall be arbitrarily called high EAH and low EAB, and controls the movement of the rod T or stem of the valve S in combination with two springs RH and RB.
The two solenoids, EAH and EAB, make it possible to maintain the mobile armature P in the high and low positions (the valve S in the closed and open position, respectively). They also make it possible to supply the energy necessary to overcome the frictions, so that the springs RB and RH can move the armature P. When the armature P moves toward the high position, the end of a rod T1, which is immovably attached to the armature, moves away from the end of the rod T and the spring RB drives the rod T upwards to position the valve in the closed position. To position the valve in the open position, the armature P is moved downwards thanks to the attraction of the solenoid EAB and to the action of the spring RH and against the spring RB, the rod T1 thus forcing back the rod T.
The to-and-fro motions of the valve are symbolized in FIG. 1 by the double arrow f.
This structure makes the valve actuators noisy by nature because of the repeated impacts of the mobile armature P against the solenoids EAH and EAB, the impacts of the end of the rod T1 against the end of the rod T, as well as the impacts of the valve against its seat.
It is known that a good control of the noise level requires resorting to a so-called closed loop automatic control system, which utilizes information on the position of the plate P. To do this, it is necessary to use a sensor for the linear position of the plate P.
In the preferred field of application of the present invention, i.e., that of internal combustion engines whose valves are associated with electromagnetic or electromechanical type actuators, it is necessary to comply with a certain number of constraints or requirements for the creation of a linear sensor, in particular:
the requirement of the control of the noise level implies knowing the position of the plate with high precision, typically a resolution on the order of 10 μm;
the temperature operating range is broad, for example, ranging from −50° C. to +150° C.; and
the range of the measurement concerns a relatively small motion, typically 8 mm, i.e., from +4 mm to −4 mm.
The environment in which the sensor is designed to operate is very difficult. The principal constraints that may have an effect on the properties and features of the sensor are as follows:
major temperature gradients capable of causing thermal drifts of the output signal of the sensor, random mechanical backlashes on the system, dilatation of the materials;
exposure to exterior electromagnetic fields capable of distorting the values of the output signal of the sensor;
exposure to a corrosive physicochemical environment: presence of moisture and of hydrocarbons, high rate of clogging, etc.
Finally, there is an additional major constraint that the sensor must comply with: It must take up little space, because it must be able to be integrated into a small available volume in the valve actuator.
In practice, a sensor comprises two principal components: a transducer generating an electric signal and a circuit, a so-called conditioner of this electric signal.
The transducer comprises at least one sensitive element which makes possible the detection of the movement of the valve S or of a piece that is secured to it, for example, the plate P, and the conversion of the associated mechanical magnitude into an electrical magnitude: an output electric signal.
The conditioner performs the shaping and electronic processing of the output signal that is generated by the transducer, so as to make it more exploitable by the above-mentioned closed loop automatic control circuits.
In practice as well, only the sensors using contactless technologies must be considered to be realistic, in view of the constraints explained above. Contactless sensors are also given preference in the automotive industry because of their reliability.
Theoretically, it would be possible to resort to various prior-art technologies, such as Hall effect sensors, capacitive sensors, inductive sensors (e.g., with a plunger core) or optical sensors.
However, these technologies are not without drawbacks. Thus, with the Hall effect sensors, the capacitive sensors or the inductive sensors, the transducer and the conditioner must be attached to each other. In fact, these sensors operate with low currents. If the transducer and the conditioner are moved away from one another, there is a risk of obtaining an excessively high signal to noise ratio and, therefore, of losing the useful signal.
The result of this is that, as concerns these types of sensors, the conditioner must be placed with the transducer in the above-mentioned thermal and corrosive physicochemical environment, which is not suitable for a valve actuator.
It is also readily recognized that the optical sensors, using optical technology, can no longer be used because of a requirement of controlling clogging incompatible with the application considered.
The object of the present invention is to eliminate these drawbacks of the prior-art devices and to respond to the needs and requirements that are demonstrated in the applications appropriate for the present invention and some of which will be mentioned.